Process for dehydrogenating secondary cyclic alcohols

ABSTRACT

In the dehydrogenation of secondary alcohols in the presence of a catalyst comprising zinc oxide and calcium carbonate at elevated temperature in the gas phase, secondary cyclic alcohols are used and the dehydrogenation is carried out in the presence of hydrogen and a catalyst whose active components comprise from 30 to 60% by weight of zinc oxide and from 40 to 70% by weight of calcium carbonate in the calcite modification.

This is the U.S. National stage application of PCT/EP97/01124 filed Mar.6, 1997.

The present invention relates to a process for dehydrogenating secondaryalcohols in the presence of a catalyst comprising zinc oxide and calciumcarbonate at elevated temperature in the gas phase.

DE-A 1,443,462 discloses a process for dehydrogenating primary andsecondary alcohols in which the alcohol used is dehydrogenated atelevated temperature in the gas phase over catalysts consistingpredominantly of zinc oxide to give the corresponding aldehyde orketone. The catalyst can contain both copper compounds and alkalineearth metals. During the dehydrogenation, ie. after hydrogen eliminationhas commenced, the hydrogen feed in the process described isdiscontinued. In particular, the dehydrogenation of cyclohexanol tocyclohexanone is described, but the yield of cyclohexanone is only81.5%. Apart from 17% of unreacted cyclohexanol, the reaction mixturecontains from 0.1 to 0.5% of hydrocarbons and 1% of higher-boilingcondensation products.

DE-B 1,296,625 describes a process for preparing cyclohexanone fromcyclohexanol contaminated with organic acids and esters at elevatedtemperatures in the presence of a zinc-containing catalyst comprisingzinc oxide-zinc carbonate or mixtures of zinc oxide-zinc carbonate withcalcium oxide-calcium carbonate or with magnesium oxide-magnesiumcarbonate. A disadvantage of this process is the excessive decrease inthe pellet hardness in long-term operation which leads to frequentreplacement of the catalyst and corresponding down times. The decreasein the pellet hardness in long-term operation results from the massivedecomposition of the carbonates by organic acids or from phasetransformations.

Acta Chim. Acad. Sci. Hung 107 (1981) 343-360, Acta Chim. Acad. Sci.Hung 97 (1978) 439-449 disclose that in the dehydrogenation ofcyclohexanol in the presence of hydrogen and catalysts comprisingelements of the eighth transition group, e.g. rhodium, nickel andplatinum, increased amounts of cracking products and formation of phenoland benzene are observed in comparison with processes not usinghydrogen. This is not observed in the dehydrogenation of aliphaticalcohols: thus DE-A 2,028,350 describes a process for dehydrogenatingaldehydes and ketones, in particular for preparing acetone and methylisobutyl ketone, over a copper-containing catalyst in the presence ofhydrogen. It may be remarked that although this reference mentionscyclohexanol as starting material among many others, there are noexperimental data on the dehydrogenation of cyclohexanol. In allprobability it would also be possible to prepare cyclohexanone by theprocess described in DE-A 2,028,350, but in the light of theabovementioned Acta Chim. Acad. Sci. Hung 107 (1981) 343-360 and alsoActa Chim. Acad. Sci. Hung 97 (1978) 439-449 one would have to expectby-products which prohibit economical use.

It is an object of the present invention to provide a process in whichcyclic ketones, in particular cyclohexanone, can be obtained in higherselectivities and yields than hitherto possible and in which theformation of cracking products and aromatic by-products is minimized.The present invention is also to make available a catalyst which inlong-term operation has a good pellet hardness, particularly in respectof compressive strength on the end face and lateral compressivestrength, so that the catalyst has to be replaced less often thanhitherto.

We have found that this object is achieved by a process fordehydrogenating secondary alcohols in the presence of a catalystcomprising zinc oxide and calcium carbonate at elevated temperature inthe gas phase, wherein secondary cyclic alcohols are used and thedehydrogenation is carried out in the presence of hydrogen and acatalyst whose active components comprise from 30 to 60% by weight ofzinc oxide and from 40 to 70% by weight of calcium carbonate in thecalcite form.

In addition, a dehydrogenation catalyst and a process for itspreparation and its use have been found.

Secondary alcohols which can be used according to the present inventionare cycloaliphatic alcohols having from 5 to 16 carbon atoms, forexample cyclopentanol, cyclohexanol, 4-methylcyclohexanol, cyclooctanol,cyclododecanol and cyclohexadecanol, preferably cyclohexanol.

The zinc oxide-containing catalyst used according to the presentinvention is a catalyst whose active components comprise from 30 to 60%by weight, preferably from 40 to 50% by weight, of zinc oxide and from40 to 70% by weight, preferably from 50 to 60% by weight, of calciumcarbonate in the calcite form.

In a preferred embodiment, the catalyst of the present invention has aBET specific surface area of from 5 to 50 m² /g, preferably from 10 to30 m² /g.

Such a catalyst is obtainable according to the present invention byprecipitation of sparingly soluble zinc and calcium compounds fromwater-soluble zinc and calcium compounds using a base and subsequentwork-up in a manner known per se, wherein

(a) the base used is a water-soluble basic carbonate,

(b) if desired, the sparingly soluble zinc and calcium compounds arefiltered off after precipitation,

(c) the zinc and calcium compounds, which may have been filtered off,are washed,

(d) the washed zinc and calcium compounds from (c) are dried to give apowder, and subsequently

(e) the powder from (d) is calcined at not above 600° C., and

(f) if desired, the calcined powder is pressed to give shaped bodies.

Water-soluble zinc and calcium salts which can be used are acetates,sulfates, nitrates, preferably nitrates such as zinc nitrate, zincacetate, zinc sulfate, calcium acetate, calcium nitrate, preferably zincnitrate and calcium nitrate. The aqueous solutions of the appropriatesalts usually have concentrations in the range from 3 to 25% by weight,preferably from 10 to 25% by weight, in particular 20% by weight.

The molar ratio of zinc to calcium is selected such that aftercalcination the active components of the catalyst comprise from 30 to60% by weight of zinc oxide and from 40 to 70% by weight of calciumcarbonate in the calcite form.

Bases used are water-soluble basic carbonates such as alkali metalcarbonates, e.g. sodium carbonate, potassium carbonate, alkali metalhydrogen carbonates, e.g. sodium hydrogen carbonate, potassium hydrogencarbonate, ammonium carbonate or ammonium hydrogen carbonate andmixtures thereof, preferably sodium carbonate, particularly preferablyin the form of its aqueous solutions generally having concentrations inthe range from 0.5 to 30, preferably from 10 to 25, gram of base/100gram of solution.

The precipitation is generally carried out at from 10 to 900° C.,preferably from 40 to 80° C. After the precipitation, the precipitatecan be filtered off if desired. The precipitate, which may have beenfiltered off, is generally washed with water, preferably until nitratecan no longer be detected by means of the nitrate ring test, and issubsequently dried at preferably from 90 to 150° C. to give a drypowder. Drying can be carried out in a static or moving bed, preferablyby spray drying.

According to the present invention, the dried powder is calcined at notabove 600° C., preferably in the range from 300 to 600° C., inparticular from 400 to 475° C., preferably in air. According toobservations up to now, prolonged heating at above 600° C. leads to theformation of the aragonite form of CaCO₃. Brief heating to above 600° C.is not detrimental to the preparation of the catalysts of the presentinvention, as long as no aragonite is formed (ie. cannot be detected bymeans of X-ray diffractometry).

After calcination, the calcined powder can, if desired, be pressed togive shaped bodies such as pellets, rings, cylinders, etc., preferablypellets.

In a preferred embodiment, the calcined powder is pressed together withgraphite, preferably with from 0.1 to 5% by weight, particularlypreferably from 1 to 2.5% by weight, in particular 2% by weight, ofgraphite, based on the total mass.

In a further preferred embodiment, the uncalcined powder from step (c)(see above) is pressed to form shaped bodies, preferably to formpellets, and the shaped bodies thus obtained are calcined as describedunder step (d).

The calcined powders and shaped bodies thus obtained can be used ascatalysts, these catalysts containing zinc oxide and calcium carbonate(in the calcite form) as active components and, if desired, graphite aspassive component.

The catalysts of the present invention have the following physicalproperties:

In a further preferred embodiment, the catalyst of the type claimed inthe present invention has a pore volume in the range from 0.10 to 0.50cm³ /g, in particular from 0.20 to 0.35 cm³ /g, at a pore diameter inthe range from 5 nm to 300 μm, where particularly preferably at least85%, preferably more than 90%, of this pore volume is associated with apore diameter in the range from 0.01 to 0.5 μm.

Particularly preferred catalysts of the type mentioned are those whichhave a compressive strength on the end face in the range from 500 to4000 N/cm², in particular from 1000 to 2500 N/cm² and a lateralcompressive strength of from 30 to 300 N, preferably from 50 to 200 N.These values can also be achieved without calcination. The importantthing is that these strength ranges are retained under operatingconditions (reaction conditions). However, this is only the case if nophase transformations occur. This condition is met by the process of thepresent invention.

The BET specific surface area is generally from 5 to 50 m² /g,preferably from 10 to 30 m² /g. The pore volume in the pore diameterrange from 5 nm to 300 μm is usually from 0.1 to 0.5 cm³ /g, preferablyfrom 0.2 to 0.35 cm³ /g, with the proviso that at least 85%, preferablymore than 90%, of this pore volume is in the pore diameter range from0.01 to 0.5 μm.

The compressive strength on the end face of the pellets is preferablyfrom 500 to 4000 N/cm², in particular from 1000 to 2500 N/cm² and thelateral compressive strength of the pellets is preferably from 30 to 300N, preferably from 50 to 200 N.

In a particularly preferred embodiment, the precipitate of sparinglysoluble zinc and calcium compounds, preferably zinc hydroxide carbonateand calcium carbonate, is washed on filter presses, the filter cake thusobtained is slurried with water and the slurry is dried by spraying in aspray dryer. The spray-dried powder obtained in this way can then befurther processed as described above.

According to the present invention, the gaseous secondary, cyclicalcohol, preferably cyclohexanol, into which from 1 to 20% by volume,preferably from 5 to 10% by volume, of hydrogen, based on the amount ofalcohol, has been mixed is brought into contact with the catalyst usedin a manner customary per se, for example in a fixed-bed reactor or in afluidized-bed reactor, preferably in a tube reactor in which thecatalyst is arranged as a fixed bed. The product mixture is usuallyworked up by distillation.

In general, the alcohol to be used is vaporized in a manner known perse, for example in a vaporizer, and the desired amount of gaseoushydrogen is then mixed in.

The temperature of the gas phase in the reaction zone is usuallyselected so as to be in the range from 200 to 500° C., preferably from300 to 450° C. In a preferred embodiment, the temperature range isselected such that a conversion in the range from 50 to 90%, preferablyfrom 65 to 75%, of alcohol is obtained. In the case of cyclohexanol asstarting compound, the temperature is selected so as to be in the rangefrom 350 to 400° C.

The pressure of the gas phase in the reaction zone is generally selectedso as to be in the range from 80 to 4000 kPa, preferably from 100 to1000 kPa.

The space velocity over the catalyst is generally selected so as to bein the range from 0.5 to 3.0, preferably from 0.6 to 2.0, liters ofalcohol per liter of catalyst per hour.

In a preferred embodiment, the hydrogen is separated from the reactionmixture leaving the reaction zone and is added to the gas mixtureentering the reaction zone.

The ketones such as cyclohexanone prepared according to the presentinvention are important large-scale industrial products. For example,cyclohexanone is customarily further used, preferably in the resultingmixture with cyclohexanol, for preparing adipic acid.

The advantage of the process of the present invention is that cyclicketones, in particular cyclohexanone, can be obtained in higher yieldsthan hitherto possible, and the formation of cracking products andaromatic by-products is minimized.

EXAMPLES Example 1

Preparation of a Calcined Catalyst (Cl₀)

For the preparation of the catalyst, two solutions are required.Solution 1 is an aqueous solution of zinc nitrate and calcium nitratehaving a concentration of 20% by weight in which the molar ratio ofzinc:calcium =1:1.6. Solution 2 is a 2M aqueous sodium carbonatesolution.

Both solutions are heated to 70° C. and pumped in parallel into aprecipitation vessel. The feed rate of the solutions is regulated suchthat a pH of 7.8±1.0 is maintained during the precipitation. Theprecipitate obtained in this parallel precipitation is filtered off andwashed with water until nitrate can no longer be detected (test usingFeSO₄ solution and concentrated H₂ SO₄, known as the nitrate ring test).The precipitate is subsequently slurried in water and spray dried. Thepowder thus obtained is heated in air for 5 hours at 450° C. and, aftercooling and addition of 2% by weight of graphite, pressed to give 5×5 mmpellets. The physical data for the catalyst ("C1₀ ") are shown in Table1.

Example 2

Dehydrogenation Using C1₀

920 g of the catalyst C1_(o) prepared as described in Example 1 wereinstalled in a tube reactor having a length of 0.6 m and an internaldiameter of 0.05 m. Via a vaporizer, 640 ml/h of liquid cyclohexanolwere fed in gaseous form into the reactor. 7 l/h of hydrogen weremetered in upstream of the reactor. The temperature of the reactionmixture in the reaction zone was maintained at 331° C. At thistemperature, the conversion based on cyclohexanol used was 70%. Thereaction mixture leaving the reactor was cooled to room temperature withrelease of hydrogen. The liquid reaction products were analyzed by gaschromatography. At a conversion of 70%, a selectivity of 99.0% and aresidue of 0.70% were obtained after 1800 hours. The catalyst (C1₁₈₀₀)had the physical properties shown in Table 1.

Comparative Example 1

Preparation of an Uncalcined Catalyst and Dehydrogenation Therewith

Calcium carbonate and zinc hydroxide carbonate were precipitated from anaqueous zinc and calcium nitrate solution by means of 2M sodiumcarbonate solution, as described in Example 1. The precipitate waswashed free of nitrate and after slurrying with H₂ O was spray dried.The dry powder thus obtained was, after addition of 2% by weight ofgraphite, pressed to give 5×5 mm pellets. The catalyst thus obtained("C2₀ ") has the physical properties indicated in Table 1. X-rayanalysis shows calcite as main component and aragonite and zinchydroxide carbonate as secondary components. 920 g of this catalystwere, as described in Example 2, tested at a cyclohexanol conversion of70%. After 1800 hours, the selectivity of the catalyst was 98.5% and theamount of residue formed was 0.88%. The catalyst (C2₁₈₀₀) had thephysical properties indicated in Table 1.

Example 3

Dehydrogenation Using C1₀ (140 h)

920 g of the catalyst (C1₀) prepared as described in Example 1 wereinstalled in a tube reactor having a length of 0.6 m and an internaldiameter of 0.05 m. Via a vaporizer, 640 ml/h of liquid cyclohexanolwere fed in gaseous form into the reactor. 7 l/h of hydrogen weremetered in upstream of the reactor. The temperature in the reaction zonewas maintained at 341° C. At this temperature, the conversion based oncyclohexanol used was 70%. The reaction mixture leaving the reactor wascooled to room temperature with release of hydrogen. The liquid reactionproducts were analyzed by gas chromatography. At a conversion of 70%, aselectivity of 98.7% and a residue of 0.80% were obtained after 140hours. The catalyst was removed from the reactor (C1₁₄₀) and had thephysical properties indicated in Table 1.

Comparative Example 2

Dehydrogenation Using C2₀, Without Hydrogen

Example 3 was repeated using catalyst (C2₀) from Comparative Example 1with the further differences that no hydrogen was used, the reactiontemperature was 323° C. and the duration of the experiment was 143hours. At a conversion of 70%, the selectivity based on cyclohexanolused was 96.6% and the residue was 2.37%. The catalyst removed from thereactor (C2₁₄₃) had the physical properties indicated in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Physical data for the catalysts                                                     BET                       Compressive                                                                         Lateral                                       surface                                                                           Total pore volume in the                                                                 Pore volume in the pore                                                                  strength                                                                            compressive         Operating           Example                                                                             area                                                                              range from 0.005 to 300                                                                  diameter range from                                                                      on the end                                                                          strength                                                                            Phases by X-ray                                                                             time                No.   [m.sup.2 ]/g                                                                      μm pore diameter [m.sup.3 /g]                                                         0.01 to 0.5 μm [%]                                                                    face N/cm.sup.2                                                                     [N]   analysis      [h]                 __________________________________________________________________________    1(C1.sub.0)                                                                         14.6                                                                              0.23       96.8       1947  94    CaCO.sub.3 calcite                                                                          0main                                                           component)                                                                    ZnO (secondary component)                                                     C graphite                        2(C1.sub.1800)                                                                      12.1                                                                              0.21       94.5       847   46    CaCO.sub.3 calcite                                                                          1800n                                                           component)                                                                    CaCO.sub.3 aragonite                                                          (secondary                                                                    component)                                                                    Zn.sub.4 CO.sub.3 (OH).sub.6                                                  H.sub.2 O                                                                     C graphite                        Comparative                                                                         12.5                                                                              0.19       84.0       1807  70    CaCO.sub.3 calcite                                                                          0main               Example 1                                   component)                        (C2.sub.0)                                  CaCO.sub.3 aragonite                                                          (secondary                                                                    component)                                                                    Zn.sub.4 CO.sub.3 (OH).sub.6                                                  H.sub.2 O                                                                     C graphite                        Comparative                                                                         17.3                                                                              0.27       72.5       664   18    CaCO.sub.3 calcite                                                                          1800n               Example 1                                   component)                        (C2.sub.1800)                               ZnO (secondary component)                                                     CaCO.sub.3 aragonite (small                                                   amount) C graphite                __________________________________________________________________________     The catalyst C1.sub.0 according to the present invention has both a highe     compressive strength on the end face and a higher lateral compressive         strength than the uncalcined catalyst C2.sub.0. Furthermore the lateral       compressive strength of the catalyst C1 only drops after 1800 hours to 49     of the initial value of the lateral compressive strength, while in the        case of comparative catalyst C2.sub.1800 containing aragonite the lateral     compressive strength drops to a  # quarter (26%) of the initial value,        which in industrial use means more down time for replacing the                aragonitecontaining catalyst.                                            

                                      TABLE 2                                     __________________________________________________________________________    Overview of the dehydrogenation experiments                                                    Amount of                                                          Temperature                                                                         Selectivity                                                                        hydrogen added                                                                        Conversion                                                                          Residue.sup.1                                  Example                                                                             [° C.]                                                                       [%]  [l/h]   [%]   [%]  Catalyst                                  __________________________________________________________________________    2     331   99.0 7       70    0.70 C1.sub.0 -->C1.sub.1800                   Comparative                                                                         331   98.5 7       70    0.88 C2.sub.0 -->C2.sub.1800                   Example 1                                                                     3     341   98.7 7       70    0.80 C1.sub.0 -->C1.sub.140                    Comparative                                                                         323   96.6 0       70    2.37 C2.sub.0 -->C2.sub.143                    Example 2                                                                     __________________________________________________________________________     .sup.1) Residue: based on the total mass of the liquid reaction mixture  

We claim:
 1. A dehydrogenation catalyst having active componentscomprising from 30 to 60% by weight of zinc oxide and from 40 to 70% byweight of calcium carbonate, wherein, prior to the catalyst'sutilization in a dehydrogenation reaction, only calcite and zinc oxidecan be detected by X-ray diffractometry, and which catalyst, in a pelletform, has a compressive strength on the end face in the range of from500 to 4000 N/cm² and a lateral compressive strength in the range offrom 30 to 300 N.
 2. The dehydrogenation catalyst defined in claim 1having a pore volume at a pore diameter in the range from 5 nm to 300 μmof from 0.1 to 0.5 cm³ /g.
 3. The dehydrogenation catalyst defined inclaim 1, wherein at least 85% of the pore volume of the catalyst has apore diameter in the range from 0.01 to 0.5 μm.
 4. A process forpreparing the dehydrogenation catalyst defined in claim 1 byprecipitation of sparingly soluble zinc and calcium compounds fromwater-soluble zinc and calcium salt solutions using a base andsubsequent work-up, comprising(a) using a water-soluble basic carbonateas the base, (b) optionally filtering off the sparingly soluble zinc andcalcium compounds after precipitation, (c) washing the precipitated zincand calcium compounds, or the zinc and calcium compounds which have beenfiltered off, (d) drying the washed zinc and calcium compounds from (c)to give a dried precursor, and subsequently (e) calcining the driedprecursor from (d) at a temperature of from 400 to not above 600° C. inthe presence of air, and (f) optionally pressing the calcined product togive shaped bodies.
 5. The process defined in claim 4, wherein afterstep (c) and before step (d) the washed zinc and calcium compounds arepressed to give shaped bodies.
 6. The process defined in claim 4,wherein the precipitate of sparingly soluble zinc and calcium compoundsis washed on filter presses, the resulting filter cake is subsequentlyslurried with water, the slurry is then dried by spraying in a spraydrier and the powder obtained is then further processed as describedunder (e) and optionally (f).
 7. A process for the dehydrogenation of asecondary cyclic alcohol, which comprises providing the catalyst definedin claim 1, and dehydrogenating said alcohol with the catalyst at atemperature of from 200 to 500° C. in the gas phase in the presence ofhydrogen.
 8. The process defined in claim 7, wherein the secondarycyclic alcohol is cyclohexanol.
 9. The process defined in claim 7,wherein the catalyst has a BET specific surface area in the range from 5to 50 m² /g.
 10. The process defined in claim 7, wherein the catalysthas a pore volume at a pore diameter in the range from 5 nm to 300 μm offrom 0.1 to 0.5 cm³ /g.
 11. The process defined in claim 10, wherein atleast 85% of the pore volume has a pore diameter in the range from 0.01to 0.5 μm.